Method of preventing ion channeling in crystalline materials

ABSTRACT

The invention discloses a method of preventing ion channelling in crystalline materials by preirradiating the material with sufficiently energetic electrons, X-rays or gamma rays to produce a sufficient density of crystal imperfections known as point defects. These defects are readily annealed away at temperatures insufficient to diffuse dopant atoms or produce a chemical or electrical effect in the material.

I United States Patent [15] 3,653,977 Gale 51 Apr. 4, 1972 [54] METHODOF PREVENTING ION [56] References Cited CHANNELING IN CRYSTALLINE UNITEDSTATES PATENTS MATERIALS 2,911,533 11/1959 Damask 250/49.5 [72]Inventor: Alfred J. Gale, Lexington, Mass. 2,968,723 1/1961 Steigerwaid..250/49.5 3,341,754 9/1967 Kellett et al. ..3l7/234 [73] Ass'gnee: aPhys'cs Corporation Buflmgton 3,383,567 5/1968 King et a] ..3 1 7/234ass.

[ Filed: p 0 9 Primary EXamir er-Ralph NllSOll Assistant Examiner-A. L,Birch [21] Appl. No; 720,023 Attorney-Francis J. Thornton [57] ABSTRACT[52] US. Cl ..l48/1.5, 317/234, 250/495,

The invention discloses a method of preventing ion chan- 29/25.3,252/623 51 I t Cl on nellmg in crystalline materials by preu'radlatmgthe material with sufficiently energetic electrons, X-rays or gamma raysto [58] Flflld of Search ..250/49.5; 148/15; 317/234, produce asufficient density of crystal imperfections known as point defects.These defects are readily annealed away at temperatures insufficient todiffuse dopant atoms or produce a chemical or electrical effect in thematerial.

7 Claims, 2 Drawing Figures I l l l 1 l0 5 ox+4 2 9 3O x I x+a Z 9 40 aE x+2 ion lmplunled 2 Gaussian Curve |n 8 Poini Defect 2 Environment 00X44 O l l X l l l 1 O 4 5 6 2 3 DEPTH MICRONS Patented April 4, 1972CONCENTRATION IONS/CM 2 Channeling Toil Dle IO Theoretical Curve MajorCrystallographic r ct n FIG. I

DEPTH -M|CRONS I I I I I KDXUfi FIG. 2 2E x04 2 m I Z x 9 so 1 IO o 40 aE [O L Ion Implanted z Gousslun Curve m 8 Point Defect 2 m Envuronment 8l0 x l l l l I I0 0 2 3 5 6 DEPTH MICRONS INVENTOR ATTORNEY METHOD OFPREVENTING ION CHANNELING IN CRYSTALLINE MATERIALS DESCRIPTION OF THEDRAWINGS FIG. 1 shows, in solid line, a typical curve of implanted iondistribution in a crystal body and, in broken line, the ion distributionfor amorphous material.

FIG. 2 shows in solid line the ion distribution in a crystalline bodyafter it has been treated in accordance with the inventron.

DISCLOSURE With the advent of further advances in solid state physicsand the need for more exotic, active, electronic devices workers in theart have turned to ion implantation as the mechanism for producing thesedevices.

Ion implantation is defined as a process in which a beam of energeticions is directed against a body of material to selectively effectelectrical and/or chemical changes in the body by causing the ions, ofthe beam, to pass into the body of treated material.

If the body of treated material is amorphous or non crystalline thehighly energetic ions thus introduced come to rest in the target as aresult of either electronic stopping or nuclear stopping or both.Electronic stopping is defined as inelastic collisions with the atomicelectrons of the target material which absorbs the energy of thebombarding ions by exciting and ionizing the target atoms. Nuclearstopping is defined as elastic collisions between the bombarding ion andthe screened nuclear field of the target atom which absorbs the ionsenergy by displacement of the target atom.

J. Lindhard and M. Scharff reported, in 1961, in the Physical Review124, 128, a theory and provided criteria whereby the distribution ofions in amorphous material may be determined. For amorphous materialthis theoretical distribution is determined to be Gaussian asillustrated in FIG. 1 as curve with a mean range determined by theenergy and a straggle (width at half maximum) determined primarily bythe relative contributions of the electronic and nuclear stopping.

This theory of Lindhard et al. has been found to be appropriate toamorphous target material but failing when applied to crystallinematerial. This failure of the theory is especially noted when the beamof incident ions is directed along an open crystallographic direction ofthe crystal.

If a crystal is examined, it will be seen that there are certaindirections in the crystal along which an undeflected ion can travelwithout encountering lattice atoms. These directions lie between planesof atoms and along tubes walled by atoms belonging to intersectingplanes and are known as channels.

Theory first predicted that occasional large angle scattering wouldprevent any impinging ion from travelling far along such channels.However, experimental results show that once an ion is aligned with achannel, either by deliberate orientation or by scattering, the ion isfocused in the channel by the nuclear charge of the lattice atoms and bythe repulsive force of two electron clouds trying to occupy the samevolume.

This focusing action permits the channelled ions to penetratesignificantly deeper into the target body before they are finallystopped by electronic stopping. However, because the electronic stoppingpower in the channel is approximately three times smaller than thatpredicted by Lindhard a deeply penetrating component or tail appears onthe ion distribution curve. This tail is shown by curve in FIG. 1.

Such channelling is undesirable since in order to get quantitativelyreproducible results, it is necessary to obtain consistency in theinitial material and to align the crystallographic directions of thetargets to within a few tenths of a degree of the desired orientationeach time. Such identity in the initial materials of the bombardedsamples is as a practical matter impossible to obtain. Moreover there isexperimental evidence that shows surface films and the effect of otherimplanted ions can also add to the irreproducibility of the distributionof channelled ions even if perfect alignment is obtained in each case.

Because of these drawbacks it is therefore desirable to eliminate asmuch as possible the probability of any ions being introduced along suchchannels.

The primary purpose of this invention therefore is to pro vide a methodwhich will prevent such channelling and provides perfectly symmetricalion distribution even in crystalline materials.

Broadly this purpose is accomplished by irradiating the crystallinetarget body with ions, electrons, X-rays or gamma rays which willproduce in the body a sufficient density of point defects that willprevent or seriously inhibit channelling during subsequent ionimplantations.

Although ions will create such point defects they are not preferredbecause they can, in semiconductor devices, produce unwanted chemical orelectrical effects or both. X- rays or gamma rays are also not preferredin that they are highly penetrating and wasteful if they are of anenergy sufficient to produce a significant number of Compton electronsin excess of the energy required to produce a lattice displacement inthe material being treated and hence a point defect.

The energy required for any material may be determined by the followingequation:

E, is the binding energy of an atom in the crystal lattice E is theminimum electron energy (threshold) required to create a point defect mis the mass of an atom of the crystal m is the rest mass of an electronv is the velocity of an electron of energy E,

c is the velocity of light.

When working with crystalline silicon this equation shows that electronbeams exceeding KeV energy should be used to produce the density ofpoint defects required to prevent channelling during a subsequent ionimplantation.

Implantation of such moderately energetic electrons can be easily andreadily achieved through the use of an electron accelerator apparatussold by High Voltage Engineering Corporation.

The invention is practised by subjecting the crystalline body to besubsequently implanted with ions to a beam of such highly energeticelectrons. The specified value of electrons used is of course dependenton the device material and thickness and for typical semiconductordevices, whose thickness is approximately 10 mils, 1 Mev electrons wouldbe used. Thinner devices would use the lower energy electrons andthicker devices would require higher energies.

These electrons transfer their energy during their passage through thecrystalline body to the lattice atoms of the body causing a slightdisplacement of the atom from its lattice site to an interstitialposition to result in a lattice vacancy and an interstitial atomseparated by distance dependent on the magnitude of the original energytransfer between the electron and the struck atom. Such disruption ofthe lattice causes at least one point defect that acts to block thecrystal channel and prevent, by electronic stopping, the channelling orpassage of subsequently implanted ions. There is however a highimprobability that any one electron, introduced into the body, willcreate more than one point defect. Thus the number of created defects issubstantially directly proportional to the number of electronsintroduced into the body being treated.

Little is known about the initial behavior of the created interstitialatoms but it is known that the created vacancies wander through thecrystal and tend to form quasi stable complexes with impurities in thecrystal rather than recombining with the interstitial atoms. Thesecreated complexes also act as point defects. In any event these channelblocking interstitial atoms or point defects cause the previouslyperfect crystal to appear to the subsequently implanted ions is found tofollow the symmetrical theoretically predicted Gaussian curve 30 of FIG.2, and the normally expected tail 40 is not existent.

These artificially created point defects however are not permanent andcan be annealed out. Fortunately they can be annealed out attemperatures much lower than those used in diffusion processing. Thevacancies will themselves anneal out at temperatures as low as 190 C.while the more dominant point defects require higher temperatures. Forexample in N- type silicon, the vacancy-oxygen and vacancy-phosphorouscomplexes have been observed to anneal out below 400 C. (the temperatureabove which affects the lifetime of the silicon material). In general,however, substantially all damage is effectively removed at temperaturesof about 300 C.

For Indium Antimonide crystals all the damage created by 1.0 Mevelectron bombardment is completely annealed out at approximately 25 C.while for Gallium Arsenide temperatures of less than 600 C. arerequired.

Thus for the subsequent ion implantation it is necessary that thecrystal body be held and maintained at a temperature below that at whichannealing occurs. In many instances it may be desirable that the body beheld at the temperature of liquid nitrogen (-1 90 C.).

In any event experimental results indicates for all semiconductorcrystalline materials that such artificially created point defectsanneal out of the crystal at or below the temperature that may berequired to activate implanted substitutional ions in the material bycausing them to become substitutional dopants. However wheninterstitially active ions are implanted in crystaline bodies, theannealing temperature need only be high enough to remove the pointdefects.

Thus such electron bombardment leaves no significant lasting effects inthe crystal while permitting the significant advantage of preventingchannelling to be obtained.

Having now described the invention it is desired that it be limited onlyby the following claims.

What is claimed is:

1. A method of treating a crystalline body to modify the physicalcharacteristics of the body consisting of maintaining said body at aselected temperature, irradiating the body with an ionizing radiationwhose energy is sufficient to create point defects in said body,bombarding said irradiated body with ions chosen to produce the selectedeffects in said body and heating said bombarded and irradiated body toremove said point defects.

2. The method of claim 1 wherein said ionizing radiation compriseselectrons.

3. The method of claim 1 wherein said temperature is below thattemperature at which said defects anneal out of said body.

4. The method of claim 3 wherein said ions are substitutionally active.

5. The method of claim 3 wherein said ions are interstially active.

6. The method of claim 1 wherein said energy is determined E is thebinding energy of an atom in the crystal lattice E is the minimumelectron energy (threshold) required to semiconductor.

lOI030 0434

2. The method of claim 1 wherein said ionizing radiation compriseselectrons.
 3. The method of claim 1 wherein said temperature is belowthat temperature at which said defects anneal out of said body.
 4. Themethod of claim 3 wherein said ions are substitutionally active.
 5. Themethod of claim 3 wherein said ions are interstially active.
 6. Themethod of claim 1 wherein said energy is determined by the equation 7.The method of claim 1 wherein said crystalline body is a semiconductor.